This invention relates to the art of digital information processing, and more particularly to a new and improved apparatus for processing coded information words to obtain selected information therefrom.
In digital information processing systems, information is transmitted between locations in the system in the form of words which are ordered sets of digits which are processed each as a unit in the system. The individual digits or groups of digits in a word can represent different types of information such as data to be processed by the system, instructions telling the system what type of operation to perform, and addresses specifying locations in the system where various imformation quantities are to be stored. The arrangement of digits or groups thereof in a word representing different types of information is known as its format. It is often desired to inspect or utilize only one of the types of information contained in the words at a given stage in the operation of the system. This poses a problem when the system has been processing words of a given format and then it receives words of a different format.
One area of use of the present invention is in digital data processing systems that emulate both new and prior art computer systems or the like. The machine language of a particular computer is the collection of all permissible machine instructions it can understand and execute, and the number, meaning and format of these machine instructions are known to vary greatly from one make or model computer to the next. Furthermore, as new and improved computing hardware is developed, it often becomes necessary to use a new and improved word format to take advantage of the improved features of the new generation equipment.
These developments, in turn, have caused significant problems for the computer user. For example, when the user of an older generation computer replaces it with a new computer he often is faced with a costly and time consuming problem of rewriting his proven and useful programs so that they can be used with the newer generation computer. A related problem faces the user of large scale computer installations including a number of computer systems which can have different machine language repertoires. A program written for one computer system of the user may not perform on another system of the same user, and this problem can increase as the user adds new equipment to his installation.
Accordingly, in recent times digital computers have been developed which have the capability of emulating other machines. The instruction words of the machine being emulated are stored in the main memory of the host machine, and in most instances the format of the instruction words of the machine being emulated is different from the format of the instruction words of the host machine. Therefore, it is necessary to rearrange the bits or groups of bits in the instruction words of the machine being emulated before they are processed by the host machine.